This invention relates generally to laser gyroscopes and more particularly to polarization dispersive structures used therein.
As described in my U.S. Pat. No. 3,741,657, assigned to the same assignee as the present invention, a laser gyroscope is shown to include a rectangular shaped laser resonator formed by providing four reflectors at the corners of such resonator. A polarization dispersive structure, made up of a Faraday rotator and a crystal rotator, is disposed in the path of "contra-rotating" laser beams in such resonator. With such an arrangement the laser beams are comprised of four radiant energy waves, one pair thereof having right hand and left hand circularly polarized waves traveling in a clockwise direction and the other pair thereof having right hand and left hand circularly polarized waves traveling in a counterclockwise direction. Because the crystal rotator and Faraday rotator provide different delays to the right hand circularly polarized waves and the left hand circularly polarized waves and because the Faraday rotator is a nonreciprocal device which provides a different delay to the clockwise traveling waves than to the counterclockwise traveling waves, each one of the four waves has a different path length and, therefore, a different frequency. Also, the propagation times of the waves are such that the pair of frequencies of the waves traveling in one direction, say the counterclockwise direction, is positioned between the frequencies of the waves traveling in the opposite, or clockwise, direction. Movement of the laser, for example, by rotation of the system about an axis perpendicular to the plane of the laser path, produces frequency shifts of the pair of waves propagating in one direction through the laser which are opposite to the frequency shifts of the waves moving in the opposite direction through the laser. This, in turn, changes the frequency separation between the lower frequencies of each said pair oppositely to a change in the frequency separation between the higher frequencies of said pair. The difference between such changes is, substantially, a linear function of the rate of said rotation and the relative sense of such difference is indicative of the direction of said rotation.
It is first noted that with the arrangement described above, in particular, the fact that an even number, i.e. four, reflectors are used to define the laser resonator, the sense of circular polarization (i.e. right hand or left hand), at any point in the laser path will not change for each revolution of the waves around the resonator. This is because each reflector may be viewed as reversing the sense of the circular polarization of the wave reflected thereby. In applications where the laser resonator is defined by an odd number of reflectors, as when V-shaped or W-shaped laser discharge tube assemblies are used in the laser gyroscope, the sense of circular polarization of any wave at any point in the laser path reverses from revolution to revolution of the waves around the resonator.
It is further noted that the relative delay between the right hand and left hand circularly polarized waves is related to the physical length of the crystal rotator used in the polarization dispersive structure. Hence, where it is desired to adjust relative delay (as in initial calibration of the gyroscope) different crystal rotators, each having a different length, must be inserted one by one in the laser path until a desired relative delay is achieved. While such a procedure may be adequate in many cases, it is obviously not desirable when adjustment of relative delay must be accomplished most quickly and easily.